The vision-impaired human eye has refractive errors which in first approximation can be described in terms of a sphere, a cylinder and an axis orientation. This is based on the assumption that the eyesight defect can be approximately corrected through a lens with a toroidal surface. This approximation is adequate to correct an error in the refraction of light rays which fall on the center of the eye pupil.
While it was customary in the past to determine the refractive errors of the human eye by relying on the subjective reaction of the patient under examination when presenting to him a plurality of optotypes of different refractive power (subjective refraction), the possibility of measuring the refractive errors of the eye has now been available for several years (objective refraction). It is possible to measure the refractive power of the eye over the entire pupil and in particular also in the peripheral areas of the pupil. The measurable errors include for example spherical aberration, coma, trefoil error, higher orders of spherical aberration, etc. The objective refraction method is based on determining the wavefront of a propagating light bundle. The functional principal of a wavefront refractor is described in DE 601 21 123 T2, which also includes a synopsis of a plurality of different variants.
It has been customary for a few years to describe the refractive errors or imaging errors of the human eye by means of so-called Zernike polynomials. The errors of the eye near the center in regard to sphere, cylinder and axis can be described through second-order Zernike polynomials. These errors are therefore often referred to as second-order errors. The errors far from the center can be described through higher-order Zernike polynomials. These errors are therefore in general also referred to as higher-order errors.
The information gained from a wavefront refractor can be used in the development of improved vision aids or improved eyesight correction methods. A well-known example for an eyesight correction method is the procedure of wave-front-guided refractive surgery. In this procedure, a volume of any desired geometry is removed from the surface of the cornea in order to correct refractive errors, including those of a higher order.
With vision aids such as for example a spectacle lens or a contact lens, this kind of correction is not generally possible at all or possible only under certain conditions. A spectacle lens has the peculiar property that the line of vision from the eye has to pass through different areas of the lens. A complete correction of higher-order errors in a spectacle lens is generally possible only for one specific direction of the line of vision. As soon as the eye looks in another direction, the correction no longer matches the higher-order errors, which lowers the vision performance. Furthermore, a complete correction of higher-order errors in a spectacle glass may lead to unacceptable distortions outside the area of correction.
However, the wave-front measurement technique can nevertheless lead of improved spectacle lenses.
The subjective refraction is conventionally performed under daylight conditions with high-contrast optotypes. This leads to refraction values which are optimized for these conditions, i.e. for a good illumination and for a high level of contrast. For many individuals, this method of refraction is not suitable for night vision or twilight vision. A wavefront measurement, on the other hand, can be performed in the dark or under mydriatic conditions. This provides the information for a much larger pupil, which opens the possibility to obtain an objective refraction result (in particular for a second-order refraction) which is also suitable for mesopic or scotopic light conditions.
Spectacle lenses, in particular progressive lenses, can have intrinsic aberrations. These intrinsic aberrations can be combined with the wavefront measurement taken for the eye, as a means to compute and manufacture improved spectacle lenses. These spectacle lenses can make it possible to at least partially correct the higher-order aberrations of the optical system constituted by the eye and the spectacle lens for at least one specific direction of the line of vision.
The determination of an improved second-order and higher-order refraction result from the wavefront measurement is known from the prior art in a multitude of variations. A concept of deriving the second-order refraction from the averaged main curvatures of the wavefronts is disclosed in U.S. Pat. No. 7,029,119.
A system for determining a correction of aberrations in an eye of a patient is described for example in EP 1 324 689 B1. The system includes a computing device which allows the correction of the data signals to be determined in such a way that, if the correction is applied to the eye, an image quality metric in an image plane of the eye is objectively optimized. In a first step, the computing device defines a search space (i.e., values that can be assumed by the coefficients), which covers several sets of coefficients (e.g., sphere, cylinder, axis, or the corresponding Zernike coefficients). In a second step, the previously selected image quality metric (e.g., Strehl ratio, variance of point image washout function, energy of the point image washout function enclosed within the Airy disc, etc.) is calculated for each of the sets of coefficients in the search space (i.e., the corresponding dioptric values for defocus and astigmatism, as well as the associated axis orientation). In a third step, the optimal value of the image quality metric is selected from all of the values of the image quality metric that were calculated in the second step, and in a fourth step, the correction is determined in conformance with one of the several sets of coefficients for which the optimal value of the image quality metric was calculated in the third step.
In their essay “Accuracy and precision of objective refraction from wavefront aberration”, which was published in Journal of Vision (2004) 4, 329-351, on Apr. 23, 2004, L. N. Thibos et al. describe a multitude of further objective methods for the determination of the refraction from a wavefront measurement.